I myself can think of a few products I would like to have for my system...Encyclopedia Magica from TSR, the full Undermountain maps and modules come to mind from Forgotten Realms DnD. Can you think of any product where the ideas and creativity of the product you would want to incorporate into running your campaign world? Would you purchase them to do so?
Have done.
The clearest examples I can think of were buying GURPS Martial Arts (G:MA) and a slew of RIFTS & other Palladium books, including one of arms & armor.
TECHNICALLY, I owned a GURPS 2Ed core rulebook and ran
some RIFTS. But I had the GURPS core book because I was in a group that regularly ran it. And I only ever ran RIFTS 3 times using Palladium’s mechanics.
Mechanics aside, G:MA had a great overview of dozens of different styles. If you understood how martial arts had been modeled in your preferred systems, you could use G:MA to diversify martial arts in them.
Likewise, the Palladium Arms & Armor book was like a Rosetta Stone for adding new weapons to other systems.
And RIFTS itself is full of nifty gonzo ideas that can be yoinked, including the setting itself- which I did for one HERO campaign.
Besides that, I loved- and used- the hell out of Primal Order and Central Casting products.
In addition, there’s a bunch of 3rd party D20 stuff I bought to supplement my D&D and D20 Modern games. Green Ronin’s
Book of the Righteous, Malhavoc’s
Hyperconscious and Perpetrated Press’
Arsenal spring right to mind, but were not alone.